I wonder if that was the end...
Once the idiots with the reins stopped hauling on its mouth, the horse did seem to calm down.
It just shows what can happen when horses are ill-prepared for such scenarios. Even on the longe, the horse looked uneasy and non-attentive, and I suspect it had not done much of that. There seemed not to be much of a connection between handler and horse at any point in the proceedings.
Not having blinkers on the bridle also went against them, as the horse was always able to see the cart behind, and this just kept upsetting him. It is possible to work some horses without blinkers, but not every horse will accept the sight of something behind.
I'm always preaching about preparation, and repetition, and how important it is to make sure the horse is comfortable with all aspects of his current training before he is asked to do something progressive. Not only does that build his confidence, but it gives the trainer a safe return to the previous level if/when things get ropy. In the event that he becomes nervous or upset, if the horse can be asked to do something he is familiar and comfortable with, he will likely settle quickly and any damage can be limited. Then the horse can be asked again when he is ready to progress.
The other main issue there was that no-one thought to attempt to inhibit the horse's vision. Every time he saw the cart, and tried to run away, all they did was try to restrain and restrict him. He was bound to fight them as a result of being held by the mouth in the face of something so fearful to him. Had any of the people had any idea, someone should have had their top off and applied a blindfold to the horse's head. As soon as he was unable to see, he would have been easier to handle until he was released from the harness. Even hands cupped behind his eyes can often be enough to focus the horse on the person at his head, preventing him from seeing behind, for long enough to make him safe.
When a horse's flight instinct kicks in, the last thing the handler should do is set themself against him and attempt to physically hold him. His instinct is telling him to flee, yet he is unable to do so. He becomes a very dangerous animal who just wants to put distance between himself and the object of his fear. If some fool is trying to hold him by the mouth, the pain will only add to his determination to get away.
Anyone can make mistakes when training horses, but the good horseman/woman should always be thinking what they may have to do quickly, in the interests of damage limitation, if things don't go according to plan.
It was a real shame what they did to that lovely horse, and I doubt if he'll ever make a driving animal after that. He'll certainly need lots of remedial training to help him do anything now.